ZHCSIN9A August 2018 – November 2018 ADS1119
PRODUCTION DATA.
The temperature measurement using a 10-kΩ thermistor is implemented using a ratiometric measurement approach to achieve best accuracy. The analog supply voltage, AVDD, is used as the excitation voltage for the thermistor in a resistor divider configuration, as well as the external reference voltage, VREF, for the ADS1119.
The relationship between output codes of the ADS1119 and the thermistor resistance, RThermistor, is derived using the following equations. Equation 10 expresses the input voltage at input AIN0 as the voltage across RThermistor, whereas Equation 11 shows how the ADC converts the voltage at AIN0 into corresponding digital codes.
Setting Equation 10 equal to Equation 11 and solving for RThermistor yields the relationship between thermistor resistance and ADC code.
Equation 13 proves that the output code and thus the accuracy of the thermistor measurement is independent of the excitation voltage. The accuracy of the reference resistor, RREF, is typically dominating the measurement accuracy in such a ratiometric circuit implementation. A high-precision, low-drift resistor is therefore required for RREF. For best performance, the value of RREF is chosen such that the ratio between RREF and RThermistor_Max equals the ratio between RThermistor_Min and RREF. Equation 14 is therefore used to calculate RREF.
At the two temperature measurement extremes, –40°C and +125°C, a typical 10-kΩ NTC exhibits a resistance of RThermistor_Max = 239.8 kΩ and RThermistor_Min = 425.3 Ω, respectively. Using Equation 14, RREF calculates to 10.1 kΩ. A 10-kΩ resistor is chosen for this example. Consequently, when using Equation 10, the voltage at the ADC input ranges from 0.13 V to 3.17 V. Thus, an ADC gain = 1 must be used for the measurement.
The microcontroller interfacing to the ADS1119 converts RThermistor into a corresponding thermistor temperature by either solving the Steinhart-Hart equation or leveraging a look-up table.